Rashi (to 8:16) explained that marital relations were now permitted after the Flood. However, the Torah records here that Noach and his family still left the Ark separately - first the males, then the females!
Ohr ha'Chayim: Even though permission was given to live together, they did not put it into practice until after they had left and parted ways.
Riva, Moshav Zekenim (Bereishis 6:18): The Torah records how they left the quarters in the Ark, i.e. the men together and the women together.
Rosh (Bereishis 8:19), R. Chaim Paltiel, based on Bereishis Rabah: They feared to have children, lest there be another Mabul. They did not have relations until HaSh-m promised not to destroy the world again. 1
Radak: The women still feared the Mabul. They did not leave until the men left.
Gur Aryeh (to 7:7) says this as well.
